Ultrasonic transducer arrangements of the above described type are generally known and are used for economically testing railroad wheels while the wheels are in their normal installed state. A train simply drives slowly over a test path that is provided with such ultrasonic transducers. One type of known ultrasonic transducer arrangement produces the ultrasonic signals by means of piezo-elements and then transmits the ultrasonic signals into the wheel set through a coupling fluid contained in a bladder. A grave disadvantage of such systems is that the bladder containing the coupling fluid often ruptures or leaks.
In addition to the above described ultrasonic transducer systems using a coupling fluid, other ultrasonic transducers are known, whereby the ultrasonic oscillations or vibrations are transmitted into the railroad wheel in a non-contacting manner. Such known ultrasonic transducers are generally based on an electrodynamic method, wherein the production of the ultrasonic pulse in the railroad wheel always produces two pulse packets or groups of pulses, which are each a segment of a wave train, that travel in opposite directions around the wheel. Both pulse packets will require the same amount of time to travel once around the wheel, because both pulses will travel the exact same path distance corresponding to the length of the circumference of the wheel. Thus, the two pulse packets, namely the right-travelling and the left-travelling pulse packets, will meet each other or coincide back at the same point at which the ultrasonic waves were produced. In other words, one pulse packet will travel clockwise around the wheel, and one pulse packet will travel counterclockwise around the wheel, but the two pulse packets will both get back to the same starting point at exactly the same moment.
In the second type of ultrasonic transducer arrangements described above, the ultrasonic pulses are produced by means of an emitter that is arranged in a testing head, and the corresponding echo signals are received by means of a receiver that is arranged immediately adjacent or even integrated into the emitter unit in the testing head. Because the two pulse packets travel around the railroad wheel in opposite directions and arrive back at the starting point together after having travelled once around the wheel, as described above, at that time they will become superimposed on one another in an area or range around the location of pulse generation, i.e. the area of the emitter unit. Depending upon the relation between the circumference of the wheel and the wavelength of the ultrasonic vibrations introduced into the wheel, the superposition of the two pulse packets will have a different characteristic. For example, the two pulse packets may be superimposed peak-on-peak, to result in a doubling of the total amplitude. On the other hand, it could occur that the two pulse packets are superimposed peak-on-trough, whereby the over-lapped or superimposed pulse packets locally cancel each other out to a zero amplitude. It is also possible that any intermediate form between a peak-to-peak or peak-to-trough superposition can occur.
The above superposition thus causes the grave disadvantage that the receiver will receive the return or turn-around signal with different signal strengths, depending on the diameter of the wheel and the wave-length of the ultrasonic vibrations. Since the amplitude of the received echo or turn-around signal is used as the reference magnitude for determining whether other signals indicate that cracks are present in the wheel, the evaluation of the turn-around signal will involve great uncertainties. These other signals are generally called error signals. As a result of the above problems, the examination of wheel sets for cracks or the like using ultrasonic transducers according to the prior art often gives erroneous results. As a consequence of such erroneous testing results, unnecessary high costs will be suffered when a wheel set that is actually defect-free is removed from a train. On the other hand, the danger exists that undetected cracks in a wheel set will result in serious operation interruptions or even breakdowns and accidents during the operation of the railroad vehicle.